This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-321749 filed on Oct. 19, 2001.
The present invention relates to a combustion condition detecting device for detecting combustion condition in an internal combustion engine.
A known combustion condition detecting device for detecting combustion condition in an internal combustion engine has a pair of opposing electrodes of a spark plug, the electrodes being connected with a current source. The combustion condition detecting device detects intensity of combustion by detecting a combustion ion current that flows between the opposing electrodes in correspondence with quantity of combustion ions existing between the opposing electrodes. This scheme is based on the fact that the quantity of the combustion ions generated in the combustion chamber changes in correspondence with the combustion condition, such as the intensity of the combustion. Thus, the combustion condition detecting device detects the combustion condition, especially an abnormal combustion condition, to control timing of discharging spark, air-fuel ratio and the like to retain favorable combustion condition, aiming at improving fuel consumption and cleaning up exhaust emissions.
U.S. Pat. No. 6,104,195 (Japanese Patent Laid-Open Publication No. 9-25867) discloses a combustion condition detecting device that applies an alternating current voltage to a pair of opposing electrodes through a transformer. Thus, combustion ions are prevented from being attracted and eliminated by the opposing electrodes, and the reduction of the detection outputs is prevented.
In this combustion condition detecting device, when the alternating current voltage is applied between the opposing electrodes, the opposing electrodes operate as a capacitor and conduct a current, which is referred to as a capacitive current hereafter. In order to inhibit the effect caused by the capacitive current, the device employs the following schemes. That is, the combustion condition detecting device monitors the fluctuation of quantity of the combustion ions by sampling current values at given phases corresponding to the alternating current voltage, as the capacitive current changes in a predetermined cycle corresponding to the alternating current voltage. Alternatively, the combustion condition detecting device monitors the fluctuation of the quantity of the combustion ions by detecting the phase differences between given phases corresponding to the alternating current voltage and phases in which the current value becomes a predetermined value.
However, the waveform of the capacitive current varies in correspondence with a degree of distortion of the waveform of the alternating current voltage applied to the opposing electrodes, causing errors in detection. In order to reduce the variations of the waveform of the capacitive current, correction may be made to the outputs of the combustion condition detecting device. However, the distortion of the waveform of the applied voltage varies due to the changes with time in characteristics of the parts constructing the combustion condition detecting device. Therefore, correction of the outputs is not capable of solving the problem fully. Alternatively, if the distortion of the waveform of the applied voltage varies due to individual differences of the parts constructing the combustion condition detecting device, it may be applicable to adjust every combustion condition detecting device individually. However, if the waveform of the applied voltage has some types of distortions, complicated adjustments between the sampled values or the detected phases and the quantity of the combustion ions are needed. Therefore, the approach of adjusting every combustion condition detecting device individually is not suitable for a mass production.
It is therefore an object of the present invention to provide a combustion condition detecting device and method for detecting combustion condition in an internal combustion engine with a high accuracy in detecting combustion ions.
According to an aspect of the present invention, a combustion condition detecting device for detecting combustion condition in a combustion chamber of an internal combustion engine has a pair of opposing electrodes disposed in the combustion chamber. The opposing electrodes are applied with an alternating current voltage through a transformer. The combustion condition detecting device detects quantity of combustion ions existing between the opposing electrodes by a current signal that represents magnitude of a current flowing between the opposing electrodes. The combustion condition detecting device has a low pass filter disposed in the primary side of the transformer. The low pass filter eliminates current components having high order frequencies of the alternating current voltage.
Since the low pass filter eliminates the current components having high order frequencies of the alternating current voltage that is outputted through the transformer and is applied to the opposing electrodes, distortion of a waveform of the alternating current voltage is reduced. As a result, the quantity of the combustion ions is detected precisely, regardless of the changes with time in the characteristics and the individual differences of the parts constructing the combustion condition detecting device.
According to another aspect of the present invention, the low pass filter is constructed by a capacitor. The capacitor discharges the current components having high order frequencies of the alternating current voltage to a grounding side.